Ultrasonic devices may be used in a wide variety of applications. For example, a hydrophone is a type of acoustic pressure sensor for calibrating an ultrasonic transducer of the type used in medical diagnosis and therapy. Calibration of the ultrasonic transducer can be achieved by directing waves from the transducer to the hydrophone. The hydrophone is operated to provide a quantitative assessment of the characteristics of the ultrasonic field that is created by the transducer in a liquid, such as water.
One type of hydrophone is referred to as a membrane hydrophone, which typically includes a thin film that is held in a taut condition by a rigid hoop. U.S. Pat. No. 4,653,036 to Harris et al. describes a membrane hydrophone having a circular sheet of polyvinylidene fluoride (PVDF) having a thickness of approximately 25 .mu.m. At the center of the PVDF sheet is a piezoelectrically active spot. A fine diameter wire is connected to the active spot at one end and is connected to a coaxial connector at the other end. The coaxial connector is fixed to the hoop that supports the sheet. Electrical signals to and from the active spot pass through the coaxial connector and an attached cable for processing by external electronics.
It is known in the industry that the diameter of the active spot affects the performance of the hydrophone. A decrease in spot size produces some desired effects, but increases the electrical impedance of the hydrophone. The thickness of the membrane also affects performance. Reducing the thickness increases the maximum frequency of the hydrophone. However, the reduction in thickness renders the hydrophone more susceptible to damage, particularly during manufacture.
The advantage of including a preamplifier has been recognized. U.S. Pat. No. 5,035,247 to Heimann describes a preamplifier that is connected to a piezoelectric sensor by a short cable. The preamplifier has an output connected to a processor unit by a longer cable. Preamplification is also described by DeReggi et al. in U.S. Pat. No. 4,433,400 and in "Piezoelectric Polymer Probe for Ultrasonic Applications," J. Acoust. Soc. Am., Vol. 69, No. 3, March 1981, pages 853-859. DeReggi et al. teach that the preamplifier may be mechanically attached at the inside diameter of the hoop that supports the piezoelectric membrane. The hoop-supported preamplifier is electrically connected by conductive epoxy material to a metallization on the membrane. The metallization extends from the preamplifier to an electrode on the active spot of the hydrophone. From the preamplifier, connection is made to a coaxial connector or the like.
A conventional membrane hydrophone typically has a diameter of approximately 100 mm. Reducing the diameter of the hydrophone may increase perturbations created by the hydrophone. On the other hand, reducing the diameter places the hoop-supported preamplifier closer to the active region, increasing the effectiveness of the preamplifier. U.S. Pat. No. 5,339,290 to Greenstein, which is assigned to the assignee of the present invention, describes a membrane hydrophone having an outer membrane supporting an interior transducer membrane that includes the piezoelectrically active region of the hydrophone. The structure of the outer membrane can be selected based upon achieving mechanical characteristics, while the structure of the transducer membrane can be selected to maximize acoustic and electrical characteristics. Greenstein cites polyimide as an acceptable material for forming the outer suspension membrane. Because it is selected for its mechanical properties, Greenstein teaches that a preamplifier may be formed on the outer suspension membrane. Thus, the preamplifier is brought closer to the active region.
While the dual-membrane hydrophone device of Greenstein decreases the signal loss by placing the amplifier closer to the active region, it does so at the expense of increasing the complexity of manufacture. However, the rationale within the transducer industry is that sacrifices related to manufacturing complexity or sacrifices related to signal loss must be made in order to achieve goals such as increasing the maximum frequency.
What is needed is an acoustic device, such as a hydrophone, and a method of forming the acoustic device such that performance is enhanced without substantial increases in manufacturing complexity.